Numerical Modelling of Graphite-Based Composite Thermal Energy Storage Unit: Effect of Numerical Variable

Thermal energy storage (TES) systems have a great potential on the providing balance of energy demand/supply, while also contributing to net-zero emissions, a reduced carbon footprint, and a greener environment. Paraffin phase change materials have emerged as a prominent material for TES applications due to its potentially high energy storage density. However, their application is significantly limited by its low thermal conductivity values. This study introduces a composite structure for thermal energy storage, utilizing paraffin as the latent heat storage material and a graphite matrix to enhance thermal conductivity for solar energy and waste heat applications. The effects of various numerical variables of mushy zone parameter, the pressure-velocity coupling, the pressure discretization scheme, and the boundary condition on the melting performance of a PCM-based thermal energy storage system were investigated within an annular storage medium, extending beyond the literature. Simulations were performed using ANSYS-Fluent, employing the enthalpy-porosity technique. The validation of the study was ensured based on the experimental setup. The primary aim of the study was to identify the numerical variables that yield the most realistic results. It was found that most closely representation of the experimental/real conditions is 105 mushy zone constant, a Coupled algorithm for the pressure-velocity coupling, and PRESTO! for the pressure discretization scheme. However, numerical variable effect was not significantly notable for the paraffin-impregnated graphite matrix storage medium. Results also indicated that graphite constrained the motion of paraffin, resulting in a uniform and homogeneous temperature distribution. It is observed that differences in numerical parameters lead to variations (0.42-16.57%) in energy storage rates, considering melting/charging times and the final temperatures of the TES system.